Spectroscopy of ${}^{28}$Na: shell evolution toward the drip line

Abstract

Excited states in ${}^{28}$Na have been studied using the ${\hat{I}}^2$-decay of implanted ${}^{28}$Ne ions at GANIL/LISE as well as the in-beam ${\hat{I}}^3$-ray spectroscopy at the NSCL/S800 facility. New states of positive (J${}^{\ddot{I}}€$=3,4${}^{+}$) and negative (J${}^{\ddot{I}}€$=1-5${}^{-}$) parity are proposed. The former arise from the coupling between 0d$\_5/2$ protons and a 0d$\_3/2$ neutron, while the latter are due to couplings with 1p$\_3/2$ or 0f$\_7/2$ neutrons. While the relative energies between the J${}^{\ddot{I}}€$=1-4${}^{+}$ states are well reproduced with the USDA interaction in the N=17 isotones, a progressive shift in the ground state binding energy (by about 500 keV) is observed between ${}^{26}$F and ${}^{30}$Al. This points to a possible change in the proton-neutron 0d$\_5/2$-0d$\_3/2$ effective interaction when moving from stability to the drip line. The presence of J${}^{\ddot{I}}€$=1-4${}^{-}$ negative parity states around 1.5 MeV as well as of a candidate for a J${}^{\ddot{I}}€$=5${}^{-}$ state around 2.5 MeV give further support to the collapse of the N=20 gap and to the inversion between the 0f$\_7/2$ and 1p$\_3/2$ levels below Z=12. These features are discussed in the framework of Shell Model and EDF calculations, leading to predicted negative parity states in the low energy spectra of the ${}^{26}$F and ${}^{25}$O nuclei.

Expérience GANIL/LISE et NSCL/S800